Matt Ridley responded via Twitter to the last article on this web site in which I tried to correct some of the errors in his Times (London) piece of 24th October. His rebuttal is appended below.

I won’t ask why he used 2014 - rather than 2016 - battery prices when he himself has said costs are declining very rapidly. Nor why he chose to illustrate his points about the Musk Gigafactory by quoting the capacity that will be available when its first phase is finished rather than when the project is complete.

He also suggests that typical 35 year lives for solar panels are very unlikely. He may not know that the world’s biggest manufacturer, Trina, now offers an insurance-based 30 year warranty.

A couple of further things are worth noting. First, the yearly output of the Gigafactory would actually supply about 5 hours of UK electricity on a typical day, not the 60 minutes he suggests. His figure is wrong by a factor of five.

Solar EROEI

Very much more importantly, I think the pernicious nonsense that solar PV does not pay back the energy used in making modules needs to be rebutted.

The academic paper to which Matt Ridley refers is one of many that have been written on the ‘energy return on energy invested’ for solar PV. The researchers are - I think - alone in now thinking that more energy goes in than ever comes out. The dozens of other people working in this field have produced results wholly in conflict with the result he chooses to pick. As with many other things Mr Ridley writes about climate matters, it would be good to see a properly academic approach to the use of external data.

I’m going to do a bit of arithmetic to try to show why Mr Ridley is vanishingly unlikely to be right that the EROEI of PV is negative. I am going to use the example of the UK. Of course in sunnier regions the numbers would be even clearer.

1, Most solar panels are made in China. Indian wholesale prices for PV are the cheapest in the world at just under 40 US cents a watt. (A watt refers to the peak output of the cell when in full sun). Let’s assume that this is the full underlying cost of the panel. No margin for the manufacturer, or the wholesaler, no transport costs or import duties.

2, A large industrial user in China, such as an integrated PV manufacturer, pays about 6 US cents per kilowatt hour for electricity. Perhaps there are some further hidden subsidies so let’s take that number down to 5 US cents. (That’s about half today’s industrial electricity price in the UK).

3, Let’s make another assumption that the entire cost of a solar panel is used to pay for electricity. This is obviously an almost absurdly conservative assumption. If each kilowatt hour costs 5 US cents, then the sales price of the panel equates to 8 kilowatt hours of electricity. (40 cents divided by 5 cents). That is the ‘energy invested’ the EROEI calculation.

4, In a reasonably good location in the UK a watt of PV produces about 1 kilowatt hour of electricity a year. If the panel last 30 years (the length of the Trina guarantee), it will produce 30 kWhs per watt of capacity. That is the ‘energy return’ of the calculation.

5, Making a series of assumptions that are clearly as favourable as possible to the case Ridley wishes to make, even in the UK the EROEI for PV is 3.75 (30 kWh/8 kWh). He is wrong to suggest PV does not make sense in energy terms.

6, And scientific progress is a wonderful thing, as Mr Ridley so eloquently shows in his writing on all non-climate matters. The energy efficiency of making PV is rising rapidly because less silicon is wasted, the cell is thinner, new materials for photon collection (such as perovskites and oligomers) are arriving and electricity efficiency is rising because of advances such as solar tracking.